Prospecting (archeology)

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Fluxgate magnetometer in the field to search for archaeological findings through geomagnetic prospecting
Video to illustrate magnetometer prospecting in the field

In archeology, prospecting (from Latin prospecto , German: looking into the distance , keeping an eye out ) means the exploration and recording of archaeological sites in a certain area, as a fundamentally non-destructive process . The methods also include archaeometry , including special bioprospection and geophysical prospection, in order to register the unknown under the surface of the earth or to take the known into consideration.

Traditional process

A simple but effective knowledge process about the existence of objects under a piece of earth is the surface inspection , also field or ground inspection or English survey . The trained eye receives information about possible sites through fault zones and disparities in the earth's surface, but also through simple excavations.

In addition, the study of written records and existing reading finds support archaeologists in their search for artefacts and buildings.

Archaeological-topographical mapping

A detailed map of the surrounding area is created using archaeological-topographic mapping . In doing so, an intensive examination of the terrain takes place, which often leads to the discovery of previously unknown terrain features, for example in dense undergrowth or overhanging rocky areas.

Methods

The methods depend on the financial possibilities as well as the requirements given by the object and the question. For economic and technical reasons, the use of bus tachymetry proved its worth until the 1990s ; nowadays, archaeological measurements are usually carried out georeferenced using a total station or even high-precision DGPS or RTK GPS measurements. The results of the survey can then be displayed and interpreted in a geographic information system (GIS).

LIDAR images give a good first impression of a site. An interpretation of the findings in the field remains inevitable. Topographic features such as rocks or walls as well as cartographic signatures (paths, embankments, etc.) must be recorded separately, graphically redrawn on the PC and integrated into the LIDAR image.

Signatures

Whichever mapping technique is chosen, from the archaeologist's point of view, a comprehensive interpretation of the terrain remains the main requirement for a topographical recording of a site. It is essential to use a uniform signature key to display the findings.

Aerial archeology

The aerial archeology evaluates aerial photographs and thus comes to new knowledge. The investigation of the soil from the air makes use of various aspects, such as:

Through the development from black and white to color photography , this method has gained considerably in accuracy and has led to an increase in quality. With false color and infrared technology one can meet the high precision requirements of archeology today. Terrestrial and aerophotogrammetry have gained in importance. The stereophotogrammetry has made a three-dimensional survey of the earth possible through photography in two planes.

Geophysical procedures

The geophysical survey uses different methods. Contrary to the general understanding in archeology, it is sometimes defined as a non-destructive process. Strictly speaking, probing and drilling do not belong to this method. This approach is mainly represented by the so-called Vienna Group.

Soil resistance measurement

Resistance map of a plowed field. The structures of a medieval moat (motte) can be seen under the field. Building structures of a bailey can be assumed at the top right.
(Archaeometry AG of the HvF Braunschweig)

The soil resistance measurement or geoelectric prospection examines the variance of the electrical conductivity of the soil due to inclusions. This basically requires two probes (non-polarizable electrodes) between which the electrical current flows. In practice, 4-pole methods such as the Wenner arrangement are used in order to neutralize the contact resistance on the four electrodes. Low-frequency alternating voltage is usually used, which eliminates the influence of contact voltages between the ground and the electrodes. This method was developed by geophysicists to locate mineral resources and raw materials and is mainly used today in groundwater prospecting.

Geomagnetic measurements

Magnetogram of two towers of the tower station Wp 10/6 on the Odenwald
Limes (Posselt & Zickgraf prospections)

For geomagnetic prospection, also called magnetic prospection or geomagnetics , proton or other nuclear magnetometers ( e.g. using rubidium , cesium or alkali vapor ) are suitable , which measure the amount of the earth's magnetic field with high precision (better than 0.1 nT) and absolute (approx. 48000 nT in Central Europe). In prospecting, however, it is usually sufficient to measure only the relative change in the earth's magnetic field , based on a base point. For this purpose, flux gate magnetometers in gradiometer suitable. They usually only measure the difference of a magnetic field component (usually the vertical one) at two different heights. The resolution is approx. 0.1 to 1 nT and is usually well suited for archaeometric prospecting. A major advantage of the gradiometer arrangement is that a correction of the magnetic field measurements is no longer necessary due to the temporal variation of the earth's magnetic field (from approx. 20 nT to more than 500 nT within a day). In the meantime, several fluxgate gradiometers are arranged in a line, so that a wide strip of the terrain can be covered during the inspection. This enables a faster measurement of large areas.

The interpretation of the measurements is mostly limited to the identification of magnetic anomalies from the magnetogram. This represents the raw data or processed data over a large area as a map of the magnetic anomalies. Further analysis of the anomalies, such as the determination of the maximum depth of the source body or its magnetization, is possible with geophysical methods, but is usually not carried out. For this purpose, gradiometer measurements are inferior to measurements of the complete magnetic field components.

Magnetic anomalies are caused by artifacts such as iron parts, slag, pottery shards, but also rotten tree stakes. In the latter case, magnetotactic bacteria are responsible for this. Geological disruptive bodies have another, mostly large-scale influence. If a gradiometer arrangement is not used, the temporal disturbances of the earth's magnetic field must also be taken into account.

Ground penetrating radar (GPR) measurements

Geophysical anomalies in a Neolithic earthwork: geoelectrics, magnetics, ground penetrating radar.
(Archaeometry AG of the HvF Braunschweig)
Use of ground penetrating radar in the Heisterburg

The ground penetrating radar (GPR-Ground Penetrating Radar) is an electromagnetic pulse reflection method, wherein the transmitted short electromagnetic pulse into the ground and received again after reflection on objects and layer boundaries or scattering at retention. In georadar measurements, a high-frequency electromagnetic wave is emitted in a frequency range between 10 and 1000 MHz, the propagation of which depends on the permittivity and electrical conductivity of the material at this frequency. Permittivity affects the speed at which the radar signal propagates, while electrical conductivity determines how much the signal is absorbed. If two different materials have the same or very similar physical properties, no signal is reflected at the boundary between the materials.

In contrast to geoelectrics and magnetics, the ground penetrating radar primarily provides a depth-resolved section through the ground below the observation line. A map of the structure sought for a depth horizon can then be created by joining neighboring sections.

literature

  • Erhard Gorys : Small handbook of archeology. Excavators and excavations, methods and terms (= dtv 3244). Deutscher Taschenbuch Verlag, Munich 1981, ISBN 3-423-03244-8 .
  • Erhard Gorys: Handbook of archeology. Excavations and excavators, methods and terms. Weltbild-Verlag, Augsburg 1989, ISBN 3-89350-120-7 .
  • Christian Bader, Werner Wild: The topographical measurement of ground monuments. In: Renate Ebersbach, Alex R. Furger (eds.): Mille fiori. Festschrift for Ludwig Berger on his 65th birthday (= research in August. Volume 25). Römermuseum, Augst 1998, ISBN 3-7151-0025-7 , pp. 227-233.
  • Wolfgang Neubauer : Magnetic prospection in archeology ( communications of the prehistoric commission of the Austrian Academy of Sciences. Volume 44). Publishing house of the Austrian Academy of Sciences, Vienna 2001, ISBN 3-7001-3009-0 , pp. 19, 160–161.
  • Dieter Vieweger : Archeology of the Biblical World (= UTB 2394). 2nd, revised edition. Vandenhoeck and Ruprecht, Göttingen 2006, ISBN 3-8252-2394-9 , pp. 116-147.
  • Jörg Bofinger : Airplane, laser, probe, spade. Remote sensing and archaeological field research using the example of the early Celtic princely seats . Regional Council Stuttgart, State Office for Monument Preservation, Esslingen a. N. 2007 ( denkmalpflege-bw.de [PDF; 5.8 MB ; accessed on April 27, 2012]).

Web links

Individual evidence

  1. ↑ Rudolf Glutz: Castle research with the theodolite, archaeological prospection on four Zug castle sites with the help of Bussolentachymetrie. In: Tugium. Volume 14, 1998, ISSN  1421-2846 , pp. 85-94.
  2. ↑ Michael Doneus, Christian Briese, Thomas Kühtreiber : Aircraft- borne laser scanning as a tool for archaeological cultural research - the case study “desert” near Mannersdorf am Leithagebirge, Lower Austria. In: Archaeological correspondence sheet. Volume 28, 2008, ISSN  0342-734X , pp. 137-156.
  3. ^ Rudolf Glutz, Klaus Grewe, Dieter Müller: Drawing guidelines for topographical plans of archaeological monument preservation. Rheinland-Verlag, Cologne 1984, ISBN 3-7927-0844-2 .
  4. ↑ Wolfgang Neubauer: Magnetic prospection in archeology. 2001, pp. 19, 160-161.